 St. Peter Sandstone is believed by geologists to have formed the coast line of a prehistoric sea stretching from Minnesota through Iowa, Illinois, and Missouri.As part of the planned rebuild of Interstate 70 throughout Missouri, this particular stretch has generated some controversy.Highway engineers have to correct the steep grade while preserving the many historic and environmentally significant sites that surround it, including Slave Rock, the Loutre River, Graham Cave, Danville Glades Conservation Area, and others.